Optical sensing sampling head

ABSTRACT

An optical sensing head which can be used with a probe and immersed in fluids and which operates to detect direct and refracted light transmission characteristics of the fluid and provide electrical signals representative thereof for further analysis. The apparatus includes a glass tube surrounded by an opaque cylindrical body having compartments to hold a light source and two photocells. The source and photocells lie in the same plane which is transverse with respect to the tube axis. An occluding pin is non-centrally disposed within the tube and is spaced from the interior walls of the tube. The body which holds the photocells and light source has openings which permit light from the source to illuminate the interior of the tube and which permits the photocells to sense light coming from within the tube, substantially all ambient light being excluded by the opaque body. The angular relationship of the openings to the photocells and source is critical, the path between one photocell and the light source being in a direct line with the occluding pin so that no light can pass directly from the source to that photocell. The other photocell, however, is disposed so that there is a direct linear light path between the source and that photocell. Thus, one cell receives direct light transmission and the other cell receives light only as a result of reflection and refraction by a fluid placed in the space in the plane containing the photocells. The apparatus includes a piston which surrounds the occluding pin within the tube and which is mounted for reciprocating movement therein to alternately block and unblock the light paths, draw fluid samples into the tube and then expel the samples, and to wipe the interior of the tube and the exterior of the occluding pin clean with sealing rings carried by the piston on each reciprocating motion. In a preferred embodiment the head is mounted on an elongated cylindrical housing so that it can be immersed to any desired depth in a body of fluid and can be cyclically driven to monitor the light transmission characteristics of the fluid while immersed.

United States Patent 91 Topol [4 1 Jan. 22, 1974 OPTICAL SENSINGSAMPLING HEAD [75] Inventor: George J. Topol, Silver Spring, Md.

[73] Assignee: Biospherics Incorporated, Rockville,

[22] Filed: May 25, 1972 [21] Appl. No.: 256,821

Primary Examiner-Har0ld A. Dixon Attorney, Agent, or FirmD. C. Roylanceet a1.

[57] ABSTRACT An optical sensing head which can be used with a probe andimmersed in fluids and which operates to detect direct and refractedlight transmission characteristics of the fluid and provide electricalsignals representative thereof for further analysis. The apparatusincludes a glass tube surrounded by an opaque cylindrical body havingcompartments to hold a light source and two photocells. The source andphotocells lie in the same plane which is transverse with respect to thetube axis. An occluding pin is non-centrally disposed within the tubeand is spaced from the interior walls of the tube. The body which holdsthe photocells and light source has openings which permit light from thesource to illuminate the interior of the tube and which permits thephotocells to sense light coming from within the tube, substantially allambient light being excluded by the opaque body. The angularrelationship of the openings to the photocells and source is critical,the path between one photocell and the light source being in a directline with the occluding pin so that no light can pass directly from thesource to that photocell. The other photocell, however, is disposed sothat there is a direct linear light path between the source and thatphotocell. Thus, one cell receives direct light transmission and theother cell receives light only as a result of reflection and refractionby a fluid placed in the space in the plane containing the photocells.The apparatus includes a piston which surrounds the occluding pin withinthe tube and which is mounted for reciprocating movement therein toalternately block and unblock the light paths, draw fluid samples intothe tube and then expel the samples, and to wipe the interior of thetube and the exterior of the occluding pin clean with sealing ringscarried by the piston on each reciprocating motion. In a preferredembodiment the head is mounted on an elongated cylindrical housing sothat it can be immersed to any desired depth in a body of fluid and canbe cyclically driven to monitor the light transmission characteristicsof the fluid while immersed.

9 Claims, 7 Drawing Figures FIG.4

FIG 5 OPTICAL SENSING SAMPLING HEAD This invention relates to anapparatus for measuring the light transmissive characteristics of a bodyof fluid.

In various situations involving fluids having optical transmissioncharacteristics which must be periodically measured or continuouslymonitored, or in circumstances in which the level of dispersedparticulate solid material in the fluid must be either measuredperiodically or continuously monitored, it is necessary to use some kindof optical device having a light source and photo sensing means in whichthe various properties of the fluid and its contents can be analyzed. Inmost circumstances, the fluid is analyzed by extracting a sample of thefluid from the tank, pipe or other container in which the fluid existsand placing the sample in a measuring device wherein it is subjected tothe necessary tests. In order to reduce the level of individualattention required to make such tests, various automatic devices havebeen proposed by which fluid samples can be taken and analyzed.

One type of device of this kind requires mounting in a pipe or tank wallwhereby it can extract samples from the larger reservoir of fluid,analyze the samples for the necessary characteristics, and return thesample to the reservoir. In principle, this is a fully satisfactorysystem,

but requires either installation when the system is installed or ratherinconvenient or expensive installation at some later date. Other systemshave been used wherein tubes, pumps or the like are employed to extractfluid samples from the reservoir and then return them to the reservoirafter the necessary tests have been made. Such systems have the severedisadvantage that large particles may not pass through the conduits ormight be altered by the transportation system itself.

Accordingly, it is an object of the present invention to provide a fluidmonitoring apparatus which can be installed temporarily by immersion ina body of the fluid to be tested, or can be installed permanently in avery inexpensive fashion.

It is a further object to provide an apparatus which measures directtransmission of light, the level of light scattered by suspendedparticles and the like, and provides information on the two forms oftransmission for analysis.

Yet another object is to provide an apparatus in which direct andscattered transmitted light is detected and correlated on a cyclicsampling basis.

Briefly described, the invention includes a tubular chamber shieldedfrom ambient light and containing a light source for illuminating theinterior of the chamber. A light sensing device is mounted in a wall ofthe chamber to receive light directly transmitted from the source. Asecond photo detecting device is mounted in an angularly spaced locationrelative to the first device and an occluding member is disposed betweenthe source and the second device so that the shadow of the occludingmember shields the second device from any direct transmission of light.Thus, the second device receives only light which has been reflected orscattered by material in the chamber. The invention also contemplatesthe incorporation of means to produce a sample fluid into the chamber,such means including a reciprocating piston which alternately draws inand ejects fluid samples. The piston can be provided with means forcyclically wiping the interior of the chamber and the oceluding deviceto prevent the accumulation of residue which might affect subsequentoperations.

In order that the manner in which the foregoing and other objects areattained in accordance with the invention can be understood in detail, aparticularly advantageous embodiment thereof will be described withreference to the accompanying drawings, which form a part of thisspecification, and wherein:

FIG. I is a perspective view, in partial section of a probeincorporating the invention;

FIG. 2 is a longitudinal section of the occluding pin and piston portionof the apparatus of FIG. 1;

FIGS. 3 and 4 are partial sections of the probe end of the apparatus ofFIG. I showing the piston in its two extreme operative positions;

FIG. 5 is a transverse section of the probe along lines 55 of FIG. 4,somewhat enlarged;

FIG. 6 is a schematic diagram of a circuit with which the light sourceand photocells of the apparatus of FIGS. 1-5 can be employed; and

FIG. 7 is a graph of typical direct and scattered light responsecharacteristics of such an apparatus.

In FIG. 1 is shown an assembled probe including a mounting plate 10 towhich is attached an elongated cylindrical tube 11 which supports andcontains the majority of the apparatus to be hereinafter described. Theprobe can be made in any convenient length, depending upon theapplication to which it is put, and depending upon the strength of thematerials used. A typical probe includes a tube approximatelycentimeters long.

A drive rod 12 extends longitudinally through the center of tube 11 andpasses through a cylindrical sleeve 13 which is attached to, or forms apart of, a cylindrical body 14 at the end of tube 11. Body 14 is provided with a central opening, to be shown and de scribed hereinafter,and three non-centrally disposed axial blind bores l5, l6 and 17. Aswill be described hereinafter, the opening identified as 15 contains alight source and openings 16 and 17 contain light responsive devices.

Sleeve 13 also has an opening for a pin 18 which extends approximatelydiametrically through the sleeve to support a pin to be describedhereinafter.

At the other end of drive shaft 12, the shaft is connected to a slideplate 20 in any convenient fashion. Plate 20 is supported forreciprocating linear movement in guide members 21 and 22, each guidemember being an inwardly facing U-shaped body. Near one edge of plate 20is provided an elongated transverse slot 23 through which a crank pin 24extends. Pin 24 is excentrically journaled in a crank wheel 25 which isconnected to a shaft, not shown, and journaled for rotary motion in amounting plate 26. The shaft to which wheel 25 is connected is coupledto a motor 27 of any convenient type.

As will be seen, rotation of motor 27 causes rotation of crank wheel 25,thereby causing crank pin 24 to repetitively describe a circle. As pin24moves in slot 23, plate 20 is caused to reciprocate in the directionsshown by arrows 28, thereby driving shaft 12 in a linear reciprocatorymanner/ A magnetic switch 30 can also be mounted on plate 26 adjacentcrank wheel 25, and a permanent magnet 31 can be embedded in theperiphery of wheel 25 so that a periodic indication of the completerotation of wheel 25 can be obtained externally of the apparatus.

FIG. 2 shows, in greater detail, the construction of drive shaft 12 andits connection to a piston 35 to which the shaft is connected withinsleeve 13. Piston 35 is an elongated cylindrical member having an anulargroove 36 in the exterior surface thereof to receive a sealing ring 37.An axial bore 38 extends inwardly from the distal end of the piston, thebore being offset from the central axis of the piston. An elongated slot39 ex tends in a direction parallel with the axis of the cylinder onopposite sides thereof, the slot opening into the bore 38 and in theother surface of the cylinder. A pin 40 resides within bore 38 and islocated by pin 18 which extends through pin 40, through slots 39, and issnugly fitted in sleeve 13 as previously mentioned in describing FIG. 1.Near the open end of the bore, a second anular groove is provided toreceive a sealing ring 42 which centrally locates pin 40 and acts towipe the exterior of the pin clean each time there is an axial relativereciprocation.

From FIGS. 1 and 2, it will be recognized that pin 40 is stationaryrelative to sleeve 13 and the remainder of the apparatus, while piston35 is reciprocated by the drive mechanism previously described.

The apparatus in the distal end of the probe assembly is more clearlyshown in FIGS. 3-5, this apparatus ineluding tubular body 14 which isprovided with axially extending openings l5, l6 and 17 to receive thelight source 45 and photocells 46 and 47, respectively.

A glass tube 48 lines the interior cylindrical bore within body 14 andsurrounds piston 35. It will be observed that sealing ring 37 wipes theinterior surface of the glass tube in the vicinity of the light sourceand photocells upon each reciprocatory motion.

The body 14 is provided with light communicating wall portions adjacenteach of the light transmitting and receiving elements, an opening 49being provided adjacent light source 45 and openings 50 and 51 beingprovided adjacent photocells 46 and 47, respectively. These openings,together with transparent walls of the glass tube, provide lighttransmitting wall portions so that light emanating from source 45 canenter the interior of the chamber formed by tube 48 and piston 35 whenthe piston is in a retracted position.

As seen in FIGS. 3 and 4, the piston is movable between a sampleejecting position, as shown in FIG. 3, and a sample collecting positionas shown in FIG. 4. The distal end of tube 48 is open to theenvironment, such as a tank of the fluid to be tested. Upon withdrawalof the piston to the left, as shown in FIG. 4, a partial vacuum iscreated by the seal provided by ring 37, causing a sample of the fluidto be drawn into the end region of tube 48. The openings 49, 50 and 51all lie in the same transverse plane with respect to the axis of theprobe, this plane also being in the region to which the sample is drawn.Thus, the light transmission from source 45 to the photocells can bedetermined.

To understand the specific nature of the light paths involved, referenceis made to FIG. in which the end of the structure is enlarged and theinterior of the chamber formed by withdrawal of piston 35 is shown withdotted shading to illustrate those portions which are normally inshadow. As indicated by arrow 53, light is transmitted directly fromsource 45 to photocell 46 and, if the fluid contained in the chamber isperfectly clear, photocell 47 would be completely in the shadow ofoccluding pin 40 and would receive no light whatever.

However, if the fluid drawn into the chamber contains suspendedparticulate material such as that indicated generally at 54, scatteredor reflected light is directed toward photocell 47 as indicated byarrows 55. Thus, the light level received at photocell 47 is a measureof the level of particulate material contained in the fluid sample, andthe relative levels oflight received by photocells 46 and 47 indicateclearly such density with inherent correction for changingcharacteristics of the light source and the transparency of tube 48.

Referring now to FIG. 6, it will be seen that a relatively simplecircuit can be employed to accept the signals resulting from the varyingphotocell outputs to produce a usable record or indication. As shown, apower source 59, which is typically a 5-volt source, is connected to alamp 45 which is the source of light previously discussed. The photocellwhich directly receives light from the lamp 45 is cell 46 which has aresistance which will be referred to as R Photocell 47 receives lightresulting from scattering due to suspended particles, direct light beingblocked by pin 40 which is schematically indicated in FIG. 6. Cell 47has a resistance which can be referred to as R The two photocells areconnected in series circuit relationship with one end of cell 47 beingconnected to the non-inverting input of a differential amplifier 63 bymeans of a resistive network which functions as both a standardizing andzeroing network. Resistor 65 and potentiometer 78 act as voltagedividers which allow the meter reading to be adjusted during instrumentcalibration to the proper levels required when sampling distilled waterand a Formazin Standard. The junction of the two cells is connected tothe inverting input of the differential amplifier through a reed switch76 and resistor 60. Switch 76 allows sampling to take place only duringthe sampling mode when piston 40 is in the withdrawn position. Acapacitor 64 is connected between the output and inverting input ofamplifier 63 to hold the last voltage applied so there will be nofluctuation of reading between samplings. The remainder of the feedbackloop consists of R in series with the proper divider resistance asselected by switch 74 which acts as a range selector. Resistors 70, 71,72 and 73 act as a signal divider by which the corresponding rangevalues of 100, 300, 1000, 3000 ppm may be selected. When the powerswitch 79 is placed in Damp (damping) position an additional l0p.fcapacitor is placed across the amplifier. This capacitor slows down theresponse of the electronic circuit during the sampling interval, thusaveraging the output signal. This prevents any abnormal sample from overinfluencing the readmgs.

The amplifier output is connected to an indicating meter 66 through aresistor 67, and the output is also connected to a potentiometer 68which can be coupled to an additional utilization device such as a stripchart recorder indicated schematically at 69.

As will be seen, the signal appearing at the junction of photocells 46and 47 is delivered to the negative input of the operational amplifier.The feedback from the amplifier output back to the input through thefeedback resistor tends to keep the input junction at virtual ground, orzero volts. This produces a voltage at the amplifier output which isproportional to the input and dependent upon the setting of the rangeswitch 74.

The response of the photocells and a typical output of a circuit such asFIG. 6 is shown in FIG. 7, the curve identified as 80 being a typicalresponse characteristic of the cell exposed only to scattered light withan increase in the concentration of the suspended solids in the samplefluid. Curve 81 represents the response of cell 46 which is exposed todirect light, and curve 82 represents a function of the ratio of thesignals produced by cells 47 and 46, this function being the outputsignal of the circuit of FIG. 6. It will be observed that this isessentially linear over a significant range of concentration, this beingthe most used range thereof. it will also be observed that neither cell,alone, is sufficiently linear or sufficiently sensitive to give the kindof information which can be obtained through proper conjunction of thetwo response characteristics.

While one advantageous embodiment has been chosen to illustrate theinvention it will be understood by those skilled in the art that variouschanges and modifications can be made therein without departing from thescope of the invention as defined in the appended claims.

What is claimed is:

1. An apparatus for measuring the light transmission characteristics ofa fluid comprising the combination of a glass tube having at least oneopen end; a generally tubular opaque body surrounding the tubularsurface of said tube, said body having openings to receive a lightsource and two photocells, and

transparent wall portions adjacent said openings to conduct lightbetween said openings and the glass tube,

said transparent wall portions being in the same plane, which planeextends generally transversely of the axis of said tube;

an elongated opaque member fixedly mounted and extending longitudinallywithin said tube, said elongated member being spaced from the innerwalls of said tube in the vicinity of the plane containing said wallportions, and being positioned to block the light path between the wallportions adjacent the light source and one photocell but allow anessentially unobstructed light path to exist between the wall portionsadjacent the light source and the other photocell; and

a piston movable longitudinally of said glass tube into and out of saidplane containing said transparent wall portions to draw samples of fluidthrough said open end of said glass tube and into and out of said planecontaining said transparent wall portions.

2. An apparatus according to claim 1 wherein said piston surrounds saidelongated opaque member, and is provided with internal and externalsealing members which contact the exterior surface of said opaque memberand the interior surface of said tube for the dual purposes of drawingfluid into the tube and wiping said surfaces clean in the vicinity ofsaid plane.

3. An immersible probe for measuring the transmission and refractionproperties of fluids comprising a tubular body having at least one openend;

an opaque pin fixedly mounted in said tubular body, said pin having anaxis parallel with the axis of said tubular body;

a piston disposed within said tubular body, and surrounding said pin,said piston being longitudinally movable relative to said tubularbody-and said pin to draw fluid into said tubular body from said oneopen end; a first light transmissive wall portion in said tubular body;

a source of light mounted adjacent said first wall portion radiallyoutwardly of said tubular body;

a second light transmissive wall portion located approximatelydiametrically opposed to said first wall portion;

a first photocell disposed adjacent said second wall portion andradially outwardly of said tubular body in a position to receive directlight transmission from said source;

a third light transmissive wall portion disposed on a line through saidlight source and perpendicular to said pin; and

a second photocell disposed adjacent said third wall portion andradially outwardly of said tubular body, said third portion and said pinbeing dimensioned and disposed to prevent direct light transmission fromsaid source to said second photocell,

said photocells being shielded by said tubular body to limit the lightreceived by said first photocell to light from said source, and to limitthe light received by said second photocell to light refracted by fluiddrawn into said tubular body.

4. A probe according to claim 3 and further comprising drive means forcyclically reciprocating said piston between a location in which lightpassage from said source to said photocells is blocked by the piston anda location in which light passage is not blocked thereby.

5. An immersible probe for measuring the transmission and refractionproperties of fluids comprising a cylinder having a longitudinalcylindrical bore extending inwardly from one end of the cylinder;

an opaque pin mounted in said bore, said pin having an axis parallelwith the axis of said cylinder;

a piston disposed within said bore and surrounding said pin, said pistonbeing longitudinally movable relative to said cylinder and said pin todraw fluid into said bore;

a first light transmissive wall portion in said cylinder;

a source of light mounted adjacent said first wall portion radiallyoutwardly of said bore;

a second light transmissive wall portion located approximately opposite'said first wall portion;

a first photocell disposed adjacent said second wall portion andradially outwardly of said bore in a position to receive direct lighttransmission from said source;

a third light transmissive wall portion disposed on a line through saidlight source and said pin;

a second photocell disposed adjacent said third wall portion andradially outwardly of said bore, said third portion and said pin beingdimensioned and disposed to prevent direct light transmission from saidsource to said second photocell, i

said photocells being shielded to limit the light received by said firstphotocell to light from said source, and to limit the light received bysaid second photocell to light refracted by fluid drawn into said bore;and

drive means for cyclically reciprocating said piston between a locationin which light passage from said source to said photocells is blocked bythe piston and a location in which light passage is not blocked thereby,and wherein said piston carries means for wiping the exterior of saidpin and the inner surfaces of said transmissive wall portions duringeach reciprocation.

6. An apparatus for measuring the light transmission characteristics ofa fluid comprising the combination of a glass tube open at at least oneend; a generally tubular opaque body surrounding said tube, said bodyhaving openings to receive a light source and two photocells, and

transparent wall portions adjacent said openings to conduct lightbetween said openings and the glass tube,

said wall portions being in the same plane, which plane extendsgenerally transversely of the axis of said tube;

an elongated opaque member fixedly mounted longitudinally in said tubein a position offset from the longitudinal axis of said tube, saidelongated opaque member being spaced from the inner walls of said tubein the vicinity of said openings, and said elongated opaque member beingpositioned to block the light path between the wall portions adjacentthe light source and one photocell but allow an essentially unobstructedlight path to exist between the wall portions adjacent the light sourceand the other photocell; and

means for causing samples of fluid to move into and out of the portionof said tube adjacent said tranparent wall portions.

7. An apparatus for measuring the light transmission characteristics ofa fluid comprising the combination of a glass tube having at least oneopen end; a generally tubular opaque body surrounding said tube, saidopaque body having openings to receive a light source and twophotocells, and transparent wall portions adjacent said openings toconduct light between each of said openings and said glass tube, all ofsaid transparent wall portions being in the same plane which extendsgenerally transversely of the axis of said glass tube; an elongatedopaque member fixedly mounted longitudinally in said glass tube in aposition offset from the longitudinal axis of said tube, said elongatedopaque member being spaced from the inner walls of said glass tube inthe vicinity of said openings, and being positioned to block the lightpath between the transparent wall portions adjacent said light sourceand one photocell, but allow an essentially unobstructed light path toexist between the wall portions adjacent the light source and the otherphotocell; and

a piston located in said tube and movable longitudinally thereof intoand out of the plane containing said transparent wall portions fordrawing samples of fluid through said open end of said glass tube andinto and out of the plane containing said transparent wall portions.

8. An apparatus according to claim 7 wherein said piston surrounds saidopaque member and is provided with internal and external sealing memberswhich contact the exterior surface of said opaque member and theinterior surface of said tube for wiping said surfaces clean in thevicinity of said plane.

9. An apparatus for measuring the light transmission characteristics ofafluid comprising the combination of an opaque member having acylindrical bore extending inwardly from one end of said member;

a transparent tube mounted in said bore, said tube having an outerdiameter equal to the inner diameter of said bore and an open andadjacent the end of said bore;

a light source mounted in said opaque member;

means defining a passageway in said opaque member for permitting lightfrom said light source to pass through the wall of said transparent tubeand into said tube;

two photocells mounted in said opaque member on an opposite side of thelongitudinal axis of said tube from said light source, said photocellsbeing angularly spaced from each other, and said photocells and saidlight source being contained in a plane generally transverse to thelongitudinal axis of said tube;

means defining two additional passageways in said opaque member forpermitting light. from the inside of said tube to pass through said tubeand illuminate said photocells;

opaque means, fixedly supported and extending longitudinally in saidtransparent tube, for permitting light emanating from said light sourceto directly illuminate one of said photocells and for preventing lightemanating from said light source to directly illuminate the other ofsaid photocells but permitting refracted light to illuminate the otherof said photocells;

piston means, mounted for reciprocal longitudinal movement in said tube,for drawing fluid into and expelling fluid from said tube through saidopen end and moving the fluid into and out of the plane containing saidlight source and said photoeells,.

said piston having means defining a cavity therein for the reception ofa portion of said opaque means whereby said piston means can move alongsaid opaque means.

1. An apparatus for measuring the light transmission characteristics ofa fluid comprising the combination of a glass tube having at least oneopen end; a generally tubular opaque body surrounding the tubularsurface of said tube, said body having openings to receive a lightsource and two photocells, and transparent wall portions adjacent saidopenings to conduct light between said openings and the glass tube, saidtransparent wall portions being in the same plane, which plane extendsgenerally transversely of the axis of said tube; an elongated opaquemember fixedly mounted and extending longitudinally within said tube,said elongated member being spaced from the inner walls of said tube inthe vicinity of the plane containing said wall portions, and beingpositioned to block the light path between the wall portions adjacentthe light source and one photocell but allow an essentially unobstructedlight path to exist between the wall portions adjacent the light sourceand the other photocell; and a piston movable longitudinally of saidglass tube into and out of said plane containing said transparent wallportions to draw samples of fluid through said open end of said glasstube and into and out of said plane containing said transparent wallportions.
 2. An apparatus according to claim 1 wherein said pistonsurrounds said elongated opaque member, and is provided with internaland external sealing members which contact the exterior surface of saidopaque member and the interior surface of said tube for the dualpurposes of drawing fluid into the tube and wiping said surfaces cleanin the vicinity of said plane.
 3. An immersible probe for measuring thetransmission and refraction properties of fluids comprising a tubularbody having at least one open end; an opaque pin fixedly mounted in saidtubular body, said pin having an axis parallel with the axis of saidtubular body; a piston disposed within said tubular body, andsurrounding said pin, said piston being longitudinally movable relativeto said tubular body and said pin to draw fluid into said tubular bodyfrom said one open end; a first light transmissive wall portion in saidtubular body; a source of light mounted adjacent said first wall portionradially outwardly of said tubular body; a seconD light transmissivewall portion located approximately diametrically opposed to said firstwall portion; a first photocell disposed adjacent said second wallportion and radially outwardly of said tubular body in a position toreceive direct light transmission from said source; a third lighttransmissive wall portion disposed on a line through said light sourceand perpendicular to said pin; and a second photocell disposed adjacentsaid third wall portion and radially outwardly of said tubular body,said third portion and said pin being dimensioned and disposed toprevent direct light transmission from said source to said secondphotocell, said photocells being shielded by said tubular body to limitthe light received by said first photocell to light from said source,and to limit the light received by said second photocell to lightrefracted by fluid drawn into said tubular body.
 4. A probe according toclaim 3 and further comprising drive means for cyclically reciprocatingsaid piston between a location in which light passage from said sourceto said photocells is blocked by the piston and a location in whichlight passage is not blocked thereby.
 5. An immersible probe formeasuring the transmission and refraction properties of fluidscomprising a cylinder having a longitudinal cylindrical bore extendinginwardly from one end of the cylinder; an opaque pin mounted in saidbore, said pin having an axis parallel with the axis of said cylinder; apiston disposed within said bore and surrounding said pin, said pistonbeing longitudinally movable relative to said cylinder and said pin todraw fluid into said bore; a first light transmissive wall portion insaid cylinder; a source of light mounted adjacent said first wallportion radially outwardly of said bore; a second light transmissivewall portion located approximately opposite said first wall portion; afirst photocell disposed adjacent said second wall portion and radiallyoutwardly of said bore in a position to receive direct lighttransmission from said source; a third light transmissive wall portiondisposed on a line through said light source and said pin; a secondphotocell disposed adjacent said third wall portion and radiallyoutwardly of said bore, said third portion and said pin beingdimensioned and disposed to prevent direct light transmission from saidsource to said second photocell, said photocells being shielded to limitthe light received by said first photocell to light from said source,and to limit the light received by said second photocell to lightrefracted by fluid drawn into said bore; and drive means for cyclicallyreciprocating said piston between a location in which light passage fromsaid source to said photocells is blocked by the piston and a locationin which light passage is not blocked thereby, and wherein said pistoncarries means for wiping the exterior of said pin and the inner surfacesof said transmissive wall portions during each reciprocation.
 6. Anapparatus for measuring the light transmission characteristics of afluid comprising the combination of a glass tube open at at least oneend; a generally tubular opaque body surrounding said tube, said bodyhaving openings to receive a light source and two photocells, andtransparent wall portions adjacent said openings to conduct lightbetween said openings and the glass tube, said wall portions being inthe same plane, which plane extends generally transversely of the axisof said tube; an elongated opaque member fixedly mounted longitudinallyin said tube in a position offset from the longitudinal axis of saidtube, said elongated opaque member being spaced from the inner walls ofsaid tube in the vicinity of said openings, and said elongated opaquemember being positioned to block the light path between the wallportions adjacent the light source and one photocell but allow anessentially unobstructed light paTh to exist between the wall portionsadjacent the light source and the other photocell; and means for causingsamples of fluid to move into and out of the portion of said tubeadjacent said tranparent wall portions.
 7. An apparatus for measuringthe light transmission characteristics of a fluid comprising thecombination of a glass tube having at least one open end; a generallytubular opaque body surrounding said tube, said opaque body havingopenings to receive a light source and two photocells, and transparentwall portions adjacent said openings to conduct light between each ofsaid openings and said glass tube, all of said transparent wall portionsbeing in the same plane which extends generally transversely of the axisof said glass tube; an elongated opaque member fixedly mountedlongitudinally in said glass tube in a position offset from thelongitudinal axis of said tube, said elongated opaque member beingspaced from the inner walls of said glass tube in the vicinity of saidopenings, and being positioned to block the light path between thetransparent wall portions adjacent said light source and one photocell,but allow an essentially unobstructed light path to exist between thewall portions adjacent the light source and the other photocell; and apiston located in said tube and movable longitudinally thereof into andout of the plane containing said transparent wall portions for drawingsamples of fluid through said open end of said glass tube and into andout of the plane containing said transparent wall portions.
 8. Anapparatus according to claim 7 wherein said piston surrounds said opaquemember and is provided with internal and external sealing members whichcontact the exterior surface of said opaque member and the interiorsurface of said tube for wiping said surfaces clean in the vicinity ofsaid plane.
 9. An apparatus for measuring the light transmissioncharacteristics of a fluid comprising the combination of an opaquemember having a cylindrical bore extending inwardly from one end of saidmember; a transparent tube mounted in said bore, said tube having anouter diameter equal to the inner diameter of said bore and an open andadjacent the end of said bore; a light source mounted in said opaquemember; means defining a passageway in said opaque member for permittinglight from said light source to pass through the wall of saidtransparent tube and into said tube; two photocells mounted in saidopaque member on an opposite side of the longitudinal axis of said tubefrom said light source, said photocells being angularly spaced from eachother, and said photocells and said light source being contained in aplane generally transverse to the longitudinal axis of said tube; meansdefining two additional passageways in said opaque member for permittinglight from the inside of said tube to pass through said tube andilluminate said photocells; opaque means, fixedly supported andextending longitudinally in said transparent tube, for permitting lightemanating from said light source to directly illuminate one of saidphotocells and for preventing light emanating from said light source todirectly illuminate the other of said photocells but permittingrefracted light to illuminate the other of said photocells; pistonmeans, mounted for reciprocal longitudinal movement in said tube, fordrawing fluid into and expelling fluid from said tube through said openend and moving the fluid into and out of the plane containing said lightsource and said photocells, said piston having means defining a cavitytherein for the reception of a portion of said opaque means whereby saidpiston means can move along said opaque means.